The present invention relates to cementing operations and, more particularly, to cement compositions comprising high alumina cement, a soluble phosphate, and cement kiln dust (“CKD”), and associated methods of use.
Cement compositions may be used in a variety of subterranean applications. An example of a subterranean application that utilizes cement compositions is primary cementing whereby pipe strings, such as casing and liners, are cemented in well bores. In performing primary cementing, a cement composition may be pumped into an annular space between the walls of a well bore and the exterior surface of the pipe string disposed therein. The cement composition sets in the annular space, thereby forming therein an annular sheath of hardened cement (i.e., a cement sheath) that supports and positions the pipe string in the well bore and bonds the exterior surface of the pipe string to the walls of the well bore. Cement compositions also may be used in remedial cementing operations, for example, to seal cracks or holes in pipe strings, to seal highly permeable zones or fractures in subterranean formations, and the like. Cement compositions also may be used in surface applications, for example, construction cementing.
Cement compositions used heretofore in subterranean applications commonly comprise Portland cement. Drawbacks may exist to using Portland cements in certain applications, however, because they are prone to corrosive attacks by carbonic acid. Other hydraulic cements also may be prone to corrosive attacks by carbonic acid. Carbonic acid may be naturally present in a subterranean formation, or it may be produced in the formation by the reaction of water and carbon dioxide when the latter is introduced into the formation, for example, during a carbon dioxide enhanced recovery operation. Carbonic acid is believed to react with calcium hydroxide that is produced by hydration of Portland cement potentially causing the deterioration of the set cement. This may be problematic, for example, because it may increase the permeability of the set cement. In some instances, the degradation of the set cement may cause loss of support for the casing and undesirable interzonal communication of fluids.
The susceptibility of some hydraulic cements (e.g., Portland cement), to degradation by carbonic acid may be especially problematic in high temperature wells (e.g., geothermal wells). The term “high temperature,” as used herein, refers to wells having a static bottom hole temperature above about 200° F. Because the high static well bore temperatures involved often coupled with brines containing carbon dioxide, these hydraulic cements may rapidly deteriorate. In geothermal wells, which typically involve high temperatures, pressures, and carbon dioxide concentrations, set cement failures have occurred in less then five years causing the collapse of well casing.
It has heretofore been discovered that cement compositions comprising water, high alumina cement, and a soluble phosphate set to form a cement that exhibits improved carbon dioxide resistance when cured in hydrothermal environments as compared to previously used cement compositions comprising Portland cement. As used herein, the term “high alumina cement” refers to cement having an alumina concentration in the range of from about 40% to about 80% by weight of the high alumina cement. The high alumina cement generally is a major component of the cost for these cement compositions. To reduce the cost of such cements compositions, other components may be included in the cement composition in addition to, or in place of, the high alumina cement. Such components may include fly ash, shale, metakaolin, micro-fine cement, and the like. “Fly ash,” as that term is used herein, refers to the residue from the combustion of powdered or ground coal, wherein the fly ash carried by the flue gases may be recovered, for example, by electrostatic precipitation.
During the manufacture of cement, a waste material commonly referred to as “CKD” is generated. “CKD,” as that term is used herein, refers to a partially calcined kiln feed which is removed from the gas stream and collected in a dust collector during the manufacture of cement. Usually, large quantities of CKD are collected in the production of cement that are commonly disposed of as waste. Disposal of the waste CKD can add undesirable costs to the manufacture of the cement, as well as the environmental concerns and costs associated with its disposal. The chemical analysis of CKD from various cement manufactures varies depending on a number of factors, including the particular kiln feed, the efficiencies of the cement production operation, and the associated dust collection systems. CKD generally may comprise a variety of oxides, such as SiO2, Al2O3, Fe2O3, CaO, MgO, SO3, Na2O, and K2O.